BIVALVIA. 241 



Generic Character. Shell transverse, subequilateral, equivalve ; ovate, elliptical, or 

 subtriangular, sometimes thin, occasionally thick and ponderous, slightly gaping 

 laterally ; surface smooth, or finely striated concentrically. Hinge with a double 

 cardinal tooth in the left valve, and a triangular space for its reception in the right, 

 with large and elongated lateral teeth on each side : a large triangular pit for the 

 cartilage. Ligament small, external. Impression by the mantle sinuated. 



Animal with its mantle freely open almost as far backward as the siphons ; margins 

 fringed ; siphons large and extensile, united to their extremities, and the orifices 

 ornamented with cirri ; foot long and strong, tongue-shaped, geniculated. 



This genus, constituting a well marked group, has not been much dismembered 

 since the time of its original establishment by Linnaeus : a division under the name of 

 Spisula, has, however, been proposed by Dr. Gray for those species which have the 

 ligament entirely hidden by the dorsal margin, and the more complete union of the 

 cartilage with the ligament, and he has reserved the name of Mactra for the species 

 which have a small shelly plate interposed between the two portions of the ligament, 

 and where it is partially visible when the valves are closed ; this distinction is so 

 very ill defined that it is not thought necessary the separation should be here adopted. 

 The most distinguishing character is the possession of a sharp diverging or chevron- 

 formed tooth, somewhat resembling the letter A reversed, and this is placed imme- 

 diately beneath the umbo before the cartilage. Some of the species have the lateral 

 teeth and the sides of the sockets, into which they are inserted, covered with rugosities 

 or ridges in a direction at right angles to these teeth ; others have these parts 

 quite smooth ; the thicker species are generally furnished with those ridges, but the 

 thinner ones are not always free or naked : these might almost constitute a sectional 

 distinction, but the same differences are observable in the genera Astarte and Cyrena. 



The species of this genus have been found in most parts of the world, living 

 principally in shallow water; they are generally buried in the sand, within a short 

 distance from the surface. 



In the fossil state they have been obtained in the Lias and Greensand formations, 

 and one is recorded as from the Carboniferous series ; but this latter is of doubtful 

 character. The species, as well as individuals, are largely developed in the newer 

 Tertiaries, and are abundant also at the present day. 



1. MACTRA GLAUCA, Born. Tab. XXIII, fig. 2, a, 6. 



MACTRA OLAUCA. Born, Test. Mus. Gees. Vind., p. 51, t. 3, figs. 11, 12, 1780. 



HELVA SEU HELVACEA. Chemn. Conch., Cab., vol. vi, p. 234, t. 23, figs. 232, 233. 



HELVACEA. Payr. Cat. Moll. Cors., p. 29, 1826. 



Phil. En. Moll. Sic., vol. i, p. 10 ; vol. ii, p. 9. 



Forb. and Hani. Hist. Brit. Moll., vol. i, p. 366, pi. 23, fig. 2, 1848. 



NEAPOLITANA. Poll. Test. Sicil., vol. i, t. 18, figs. 13, 1791. 



32 



